This invention relates in general to displaying information on graphics display screen, and more specifically to increasing the amount of informaion which can be displayed upon a single graphics display screen.
Graphic display screens are made up of thousands of small elements called picture elements, or pixels. In general, these pixels are arranged in horizontal rows. For example, a commonly used display format employs 1024 such rows with 1280 pixels in each row. In a color display, each pixel is made up of three fluorescent elements that, when acted upon by three separate electron beams, produce red, green, and blue light of variable intensities. Consequently, the color and intensity of each pixel is determined by information transmitted by the three electron beams. In a typical digital graphics display, the information that controls the electron beams, and hence the colors of the various pixels, is provided by a color look-up table within the graphics display circuitry. This color look-up table supplies three parallel sequences of n-bit digital numbers. These parallel sequences are converted to three equivalent analog signals for the purpose of controlling the three electron beams. In typical systems, n is generally 4 or 8.
The color lookup table is controlled by a set of bits of information supplied from a memory circuit. The memory circuit has p bit planes, where p is the number of bits of information that control each pixel. Each bit planes comprises memory space corresponding to the number of pixels on the graphics display screen. For example, a bit plane for a 1024.times.1280 pixel display will include 1,310,720 memory elements. In a typical display application, each bit plane, or group of bit planes, represents a specific type of information which must be displayed on the screen. The bit planes are prioritized according to the importance of the informaton. For instance, a map might be shown upon the graphics display screen as a combination of background coloring, contour lines and specific landmarks or sites of interest. The background color and its associated bit planes will have the lowest priority. The contour lines and their associated bit planes will have intermediate priority, and the specific landmarks and their associated bit planes will have the highest priority. The contents of the color look-up table will assure that information on a higher prioritized bit plane, or group of bit planes, will overlay, or cover, information on a lower prioritized bit plane or group of bit planes.
A user of a graphics display system might desire several different types of operations to allow flexibility in the system. For instance, operators commonly desire large numbers of independent bit planes, or groups of bit planes, to allow manipulation of many classes of data. These classes might include map features such as boundaries, roadways, railroads, transmission lines, and buildings. Preferably, changes in one class of data should be effectuated without disturbing the other classes of data.
Another common desire of graphics display users is the ability to move lines, symbols, or alpha-numerical characters in an animated fashion without altering or otherwise disturbing the background map. Two groups of data are displayed alternatively to generate the animated characters without altering the background map. This process is referred to as doublebuffering or ping-ponging. Double-buffering is accomplished by connecting two or more sets of bit planes to the lookup table. When information is being read from one set by the lookup table, new information relating to the animated characters are written to the other set by the original data source.
Another common desire of graphics display users is the ability to temporarily delete all data in a given class or classes without disturbing anything else on the display; that is, making certain information (class of information) particularly noticeable by removing other unwanted information. Other applications might require all symbols of a given class to blink (that is, disappear and reappear with a given frequency) or to have only one or several symbols of a given class blink (blinking is often controlled by a blink plane associated with the bit planes that controls the given class of data) to emphasize those symbols of classes. The user also might want to have all objects of a given class appear in a designated color or to appear in the original colors but with a change in intensity.
These desired functions are currently performed using multiple bit planes. Multiple bit planes store the data, or information, and generate the electronic signals carrying the information to the color lookup table. Multiplex switches select alternative sets of bit plane data in the double buffering process, and control the color lookup table. When double-buffering is required, all bit planes must be duplicated even if most of the data is constant and only a small part of it is changing. Additionally, the lookup table becomes very complex when a large number of bit planes are required for graphics displays having many required classes of information. These are the principal limitations of current equipment.
The most complex systems currently available illustrate the limitations of conventional systems. These complex systems convert 12-bit pixel descriptions into three sets of 8 bits each for the red, green and blue signals. The systems must provide 2.sup.12 .times.24=98,304 storage locations in the color lookup table. This number of storage locations may practically be incorporated within graphics display computers. However, larger pixel descriptions would become technically impractical, or if technically feasible, then extremely expensive. For instance, 16-bit pixel descriptions would require 2.sup.16 .times.24=1,572,864 storage locations, and 24-bit pixel descriptions would require 2.sup.24 .times.24=4.027(10).sup.8 storage locations in the color lookup table. Such color look up tables are not economically feasible.